1. Field of the Invention
The present invention relates to a zoom lens barrel.
2. Description of Related Art
Compact cameras currently in the market include such cameras that are using zoom lens barrels having a zoom ratio of three magnifications or thereabout. Some of the zoom lens barrels of this kind are of a three-stage draw-out lens-barrel structure which is arranged to make the whole lens barrel length approximately equal to the thickness of the camera body when the camera is not used for photo-taking.
The three-stage draw-out lens-barrel structure enables the camera to be arranged to have a so-called full-flat body when the camera is not used for photo-taking, despite the use of such a high zoom ratio of the zoom lens barrel as three magnifications. Three examples of zoom lens barrels of the three-stage draw-out lens-barrel structure were disclosed respectively in Japanese Laid-Open Patent Applications No. HEI 7-27963, No. HEI 8-211278 and No. HEI 8-248461, which hereinafter will be referred to as examples 1, 2 and 3 of conventional arrangement, respectively.
The zoom lens barrel in the example 1 of conventional arrangement is composed of a fixed tube which is fixedly mounted on a camera body, a first movable tube (a zoom driving ring) which is arranged within the fixed tube to be drawable out of the fixed tube, a second movable tube (a zoom cam ring) which is arranged within the first movable tube to be drawable out of the first movable tube, and a third movable tube (forward motion tube) which is arranged within the second movable tube to be drawable out of the second movable tube. Further, a rectilinear lead tube is disposed between the first and second movable tubes while a rectilinear tube is disposed on the inner side, i.e., on the optical axis side, of the third movable tube. The rectilinear lead tube is arranged to move along with the zoom driving ring in the optical axis direction while preventing the rotating motion of the third movable tube.
The zoom lens barrel in the example 2 of conventional arrangement is composed of a fixed tubular member, a first draw-out tube which is helicoid-coupled with the fixed tubular member, a second draw-out tube which is helicoid-coupled with the first draw-out tube, a third draw-out tube which is helicoid-coupled with the second draw-out tube, a rotary member which is arranged to be movable together with the first draw-out tube in the optical axis direction and to be rotatable relative to the first draw-out tube in such a way as to cause the second draw-out tube to rotate, and a rectilinear guide member which is arranged to be movable in the optical axis direction together with the second draw-out tube and to be rotatable relative to the second draw-out tube. The rotary member is thus arranged to move in the optical axis direction while transmitting a rotative driving force to the second draw-out tube.
The zoom lens barrel in the example 3 of conventional arrangement is composed of a fixed tube, a rear tube which is arranged inside of the fixed tube to be drawn out therefrom while being rotated, a rectilinear cam tube which is arranged on the inner side of the rear tube to move in the optical axis direction together with the rear tube, a rotary tube which is arranged on the inner side of the rectilinear cam tube to move together with the rear tube in the optical axis direction while being rotated, a rectilinear key which is arranged on the inner side of the rotary tube to move in the optical axis direction together with the rectilinear cam tube, an intermediate tube which is arranged on the inner side of the rotary tube to be drawn out while being rotated, a rectilinear member which is arranged on the inner side of the intermediate tube to move in the optical axis direction together with the intermediate tube, and a front tube which is arranged between the intermediate tube and the rectilinear member to be drawn out in the optical axis direction.
With the conventional zoom lens barrel arranged in the above-stated manner, a camera can be arranged to have a full flat body when the camera is not used for photo-taking and also to permit reduction in thickness thereof. However, the above examples 1, 2 and 3 of conventional arrangement have the following problems.
The zoom lens barrel in the example 1 of conventional arrangement makes it necessary to have a space solely for the rectilinear lead tube between the first and second movable tubes. This space hinders reduction in size of the lens barrel in the direction of its diameter. Another problem lies in that the rectilinear lead tube must be provided with a helical through-hole, which lowers the strength of the lens barrel.
In the zoom lens barrel in the example 2 of conventional arrangement, the rotary member, which corresponds to the rectilinear lead tube in the example 1 of conventional arrangement, is spatially allocated closer to the optical axis to facilitate reduction in size of the lens barrel in the direction of its diameter. However, the lead of the rotary member must be strictly arranged to have the rectilinear guide member move together with the second draw-out tube without rotating relative to the fixed tubular member. This requirement not only necessitates complex assembly work but also causes an increase in cost of manufacture.
The zoom lens barrel in the example 3 of conventional arrangement does not have any helical through-hole in the rectilinear key, which corresponds to the rectilinear lead tube in the example 1 of conventional arrangement. However, the rectilinear cam tube and the rotary tube are additionally included in the zoom lens barrel in the example 3 of conventional arrangement. These additional members cause an increase in diameter of the lens barrel.